scene/workspacescene: don't check direct scanout candidates for a pixmap
We don't need a pixmap for direct scanout, and the drm backend destroys the pixmap when direct scanout is successful... so this check created a loop of direct scanout working and not working, and worse, the client reallocating its buffers each time.
BUG: 485639 BUG: 485730 BUG: 485712 CCBUG: 477016
(cherry picked from commit fba948b3)